lorax - 1:08 am on Mar 26, 2013 (gmt 0)
Sub-domains and sub-directories are pretty much even in value as far as Google goes. Matt C said so in one of his videos (though I don't have it at hand right now). As long as your blog is on the same topic line as the main site, the sub-domain should be fine. Just make sure the hyperlinks are natural and useful and your posts are not spammy. IMHO of course.